Flat-panel displays are of great interest as information displays for computing, entertainment, and communications. For example, electroluminescent (EL) emitters have been known for some years and have recently been used in commercial display devices. Such displays employ both active-matrix and passive-matrix control schemes and can employ a plurality of subpixels. Each subpixel contains an EL emitter and a drive transistor for driving current through the EL emitter. The subpixels are typically arranged in two-dimensional arrays with a row and a column address for each subpixel, and having a data value associated with the subpixel. Subpixels of different colors, such as red, green, blue, and white are grouped to form pixels. EL displays can be made using various emitter technologies, including coatable-inorganic light-emitting diode, quantum-dot, and organic light-emitting diode (OLED).
Electroluminescent (EL) flat-panel display technologies, such as organic light-emitting diode (OLED) technology, provide benefits in color gamut, luminance, and power consumption over other technologies such as liquid-crystal display (LCD) and plasma display panel (PDP). However, EL displays suffer from performance degradation over time. In order to provide a high-quality image over the life of the display, this degradation must be compensated for. Furthermore, OLED displays suffer from visible nonuniformities across a display. These nonuniformities can be attributed to both the EL emitters in the display and, for active-matrix displays, to variability in the thin-film transistors used to drive the EL emitters.
The light output of an EL emitter is roughly proportional to the current through the emitter, so the drive transistor in an EL subpixel is typically configured as a voltage-controlled current source responsive to a gate-to-source voltage Vgs. Source drivers similar to those used in LCD displays provide the control voltages to the drive transistors. Source drivers can convert a desired code value into an analog voltage to control a drive transistor. The relationship between code value and voltage is typically non-linear, although linear source drivers with higher bit depths are becoming available. Although the nonlinear code value-to-voltage relationship has a different shape for OLEDs than the characteristic LCD S-shape (shown in e.g. U.S. Pat. No. 4,896,947), the source driver electronics required are very similar between the two technologies. In addition to the similarity between LCD and EL source drivers, LCD displays and EL displays are typically manufactured on the same substrate: amorphous silicon (a-Si), as taught e.g. by Tanaka et al. in U.S. Pat. No. 5,034,340. Amorphous Si is inexpensive and easy to process into large displays.
Degradation Modes
Amorphous silicon, however, is metastable: over time, as voltage bias is applied to the gate of an a-Si TFT, its threshold voltage (Vth) shifts, thus shifting its I-V curve (Kagan & Andry, ed. Thin-film Transistors. New York: Marcel Dekker, 2003. Sec. 3.5, pp. 121-131). Vth typically increases over time under forward bias, so over time, Vth shift will, on average, cause a display to dim.
In addition to a-Si TFT instability, modern EL emitters have their own instabilities. For example, in OLED emitters, over time, as current passes through an OLED emitter, its forward voltage (Voled) increases and its efficiency (typically measured in cd/A) decreases (Shinar, ed. Organic Light-Emitting Devices: a survey. New York: Springer-Verlag, 2004. Sec. 3.4, pp. 95-97). The loss of efficiency causes a display to dim on average over time, even when driven with a constant current. Additionally, in typical OLED display configurations, the OLED is attached to the source of the drive transistor. In this configuration, increases in Voled will increase the source voltage of the transistor, lowering Vgs and thus, the current through the OLED emitter (Ioled), and therefore causing dimming over time.
These three effects (Vth shift, OLED efficiency loss, and Voled rise) cause each individual OLED subpixel to lose luminance over time at a rate proportional to the current passing through that OLED subpixel. (Vth shift is the primary effect, Voled shift the secondary effect, and OLED efficiency loss the tertiary effect.) Therefore, as the display dims over time, those subpixels that are driven with more current will fade faster. This differential aging causes objectionable visible burn-in on displays. Differential aging is an increasing problem today as, for example, more and more broadcasters continuously superimpose their logos over their content in a fixed location. Typically, a logo is brighter than content around it, so the pixels in the logo age faster than the surrounding content, making a negative copy of the logo visible when watching content not containing the logo. Since logos typically contain high-spatial-frequency content (e.g. the AT&T globe), one subpixel can be heavily aged while an adjacent subpixel is only lightly aged. Therefore, each subpixel must be independently compensated for aging to eliminate objectionable visible burn-in.
Moreover, some transistor technologies, such as low-temperature polysilicon (LTPS), can produce drive transistors that have varying mobilities and threshold voltages across the surface of a display (Kuo, Yue, ed. Thin Film Transistors: Materials and Processes, vol. 2: Polycrystalline Thin Film Transistors. Boston: Kluwer Academic Publishers, 2004. pg. 412). This produces objectionable nonuniformity. Further, nonuniform OLED material deposition can produce emitters with varying efficiencies, also causing objectionable nonuniformity. These nonuniformities are present at the time the panel is sold to an end user, and so are termed initial nonuniformities, or “mura.” FIG. 11A shows an example histogram of subpixel luminance exhibiting differences in characteristics between subpixels. All subpixels were driven at the same level, so should have had the same luminance. As FIG. 11A shows, the resulting luminances varied by 20 percent in either direction. This results in unacceptable display performance.
Prior Art
It has been known to compensate for one or more of the three aging effects. Similarly, it is known in the prior art to measure the performance of each pixel in a display and then to correct for the performance of the pixel to provide a more uniform output across the display.
Considering Vth shift, the primary effect and one which is reversible with applied bias (Mohan et al., “Stability issues in digital circuits in amorphous silicon technology,” Electrical and Computer Engineering, 2001, Vol. 1, pp. 583-588), compensation schemes are generally divided into four groups: in-pixel compensation, in-pixel measurement, in-panel measurement, and reverse bias.
In-pixel Vth compensation schemes add additional circuitry to each subpixel to compensate for the Vth shift as it happens. For example, Lee et al., in “A New a-Si:H TFT Pixel Design Compensating Threshold Voltage Degradation of TFT and OLED”, SID 2004 Digest, pp. 264-274, teach a seven-transistor, one-capacitor (7T1C) subpixel circuit which compensates for Vth shift by storing the Vth of each subpixel on that subpixel's storage capacitor before applying the desired data voltage. Methods such as this compensate for Vth shift, but they cannot compensate for Voled rise or OLED efficiency loss. These methods require increased subpixel complexity and increased subpixel electronics size compared to the conventional 2T1C voltage-drive subpixel circuit. Increased subpixel complexity reduces yield, because the finer features required are more vulnerable to fabrication errors. Particularly in typical bottom-emitting configurations, increased total size of the subpixel electronics increases power consumption because it reduces the aperture ratio, the percentage of each subpixel which emits light. Light emission of an OLED is proportional to area at a fixed current, so an OLED emitter with a smaller aperture ratio requires more current to produce the same luminance as an OLED with a larger aperture ratio. Additionally, higher currents in smaller areas increase current density in the OLED emitter, which accelerates Voled rise and OLED efficiency loss.
In-pixel measurement Vth compensation schemes add additional circuitry to each subpixel to permit values representative of Vth shift to be measured. Off-panel circuitry then processes the measurements and adjusts the drive of each subpixel to compensate for Vth shift. For example, Nathan et al., in U.S. Patent Application Publication No. 2006/0273997, teach a four-transistor pixel circuit which permits TFT degradation data to be measured as either current under given voltage conditions or voltage under given current conditions. Nara et al., in U.S. Pat. No. 7,199,602, teach adding an inspection interconnect to a display, and adding a switching transistor to each pixel of the display to connect it to the inspection interconnect. Kimura et al., in U.S. Pat. No. 6,518,962, teach adding correction TFTs to each pixel of a display to compensate for EL degradation. These methods share the disadvantages of in-pixel Vth compensation schemes, but some can additionally compensate for Voled shift or OLED efficiency loss.
In-pixel measurement Vth compensation schemes add circuitry around a panel to take and process measurements without modifying the design of the panel. For example, Naugler et al., in U.S. Patent Application Publication No. 2008/0048951, teach measuring the current through an OLED emitter at various gate voltages of a drive transistor to locate a point on precalculated lookup tables used for compensation. However, this method requires a large number of lookup tables, consuming a significant amount of memory. Further, this method does not recognize the problem of integrating compensation with image processing typically performed in display drive electronics. It also does not recognize the limitations of typical display drive hardware, and so requires a timing scheme which is difficult to implement without expensive custom circuitry.
Reverse-bias Vth compensation schemes use some form of reverse voltage bias to shift Vth back to some starting point. These methods cannot compensate for Voled rise or OLED efficiency loss. For example, Lo et al., in U.S. Pat. No. 7,116,058, teach modulating the reference voltage of the storage capacitor in an active-matrix pixel circuit to reverse-bias the drive transistor between each frame. Applying reverse-bias within or between frames prevents visible artifacts, but reduces duty cycle and thus peak brightness. Reverse-bias methods can compensate for the average Vth shift of the panel with less increase in power consumption than in-pixel compensation methods, but they require more complicated external power supplies, can require additional pixel circuitry or signal lines, and may not compensate individual subpixels that are more heavily faded than others.
Considering Voled shift and OLED efficiency loss, U.S. Pat. No. 6,995,519 by Arnold et al. is one example of a method that compensates for aging of an OLED emitter. This method assumes that the entire change in emitter luminance is caused by changes in the OLED emitter. However, when the drive transistors in the circuit are formed from a-Si, this assumption is not valid, as the threshold voltage of the transistors also changes with use. The method of Arnold will thus not provide complete compensation for subpixel aging in circuits wherein transistors show aging effects. Additionally, when methods such as reverse bias are used to mitigate a-Si transistor threshold voltage shifts, compensation of OLED efficiency loss can become unreliable without appropriate tracking/prediction of reverse bias effects, or a direct measurement of the OLED voltage change or transistor threshold voltage change.
Alternative methods for compensation measure the light output of each subpixel directly, as taught e.g. by Young et al. in U.S. Pat. No. 6,489,631. Such methods can compensate for changes in all three aging factors, but require either a very high-precision external light sensor, or integrated light sensors in each subpixel. An external light sensor adds to the cost and complexity of a device, while integrated light sensors increase subpixel complexity and electronics size, with attendant performance reductions.
Regarding initial-nonuniformity compensation, U.S. Patent Application Publication No. 2003/0122813 by Ishizuki et al. discloses a display panel driving device and driving method for providing high-quality images without irregular luminance. The light-emission drive current flowing is measured while each pixel successively and independently emits light. Then the luminance is corrected for each input pixel data based on the measured drive current values. According to another aspect, the drive voltage is adjusted such that one drive current value becomes equal to a predetermined reference current. In a further aspect, the current is measured while an off-set current, corresponding to a leak current of the display panel, is added to the current output from the drive voltage generator circuit, and the resultant current is supplied to each of the pixel portions. The measurement techniques are iterative, and therefore slow. Further, this technique is directed at compensation for aging, not for initial nonuniformity.
U.S. Pat. No. 6,081,073 by Salam describes a display matrix with a process and control means for reducing brightness variations in the pixels. This patent describes the use of a linear scaling method for each pixel based on a ratio between the brightness of the weakest pixel in the display and the brightness of each pixel. However, this approach will lead to an overall reduction in the dynamic range and brightness of the display and a reduction and variation in the bit depth at which the pixels can be operated.
U.S. Pat. No. 6,473,065 by Fan describes methods of improving the display uniformity of an OLED. In this method, the display characteristics of all organic-light-emitting-elements are measured, and calibration parameters for each organic-light-emitting-element are obtained from the measured display characteristics of the corresponding organic-light-emitting-element. The calibration parameters of each organic-light-emitting-element are stored in a calibration memory. The technique uses a combination of look-up tables and calculation circuitry to implement uniformity correction. However, the described approaches require either a lookup table providing a complete characterization for each pixel, or extensive computational circuitry within a device controller. This is likely to be expensive and impractical in most applications.
U.S. Pat. No. 7,345,660 by Mizukoshi et al. describes an EL display having stored correction offsets and gains for each subpixel, and having a measurement circuit for measuring the current of each subpixel. While this apparatus can correct for initial nonuniformity, it uses a sense resistor to measure current, and thus has limited signal-to-noise performance. Furthermore, the measurements required by this method can be very time-consuming for large panels.
U.S. Pat. No. 6,414,661 by Shen et al. describes a method and associated system that compensates for long-term variations in the light-emitting efficiency of individual organic light emitting diodes in an OLED display device by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel and derives a correction coefficient that is applied to the next drive current for each pixel. This patent describes the use of a camera to acquire images of a plurality of equal-sized sub-areas. Such a process is time-consuming and requires mechanical fixtures to acquire the plurality of sub-area images.
U.S. Patent Application Publication No. 2005/0007392 by Kasai et al. describes an electro-optical device that stabilizes display quality by performing correction processing corresponding to a plurality of disturbance factors. A grayscale characteristic generating unit generates conversion data having grayscale characteristics obtained by changing the grayscale characteristics of display data that defines the grayscales of pixels with reference to a conversion table whose description contents include correction factors. However, their method requires a large number of LUTs, not all of which are in use at any given time, to perform processing, and does not describe a method for populating those LUTs.
U.S. Pat. No. 6,989,636 by Cok et al. describes using a global and a local correction factor to compensate for nonuniformity. However, this method assumes a linear input and is consequently difficult to integrate with image-processing paths having nonlinear outputs.
U.S. Pat. No. 6,897,842 by Gu describes using a pulse width modulation (PWM) mechanism to controllably drive a display (e.g., a plurality of display elements forming an array of display elements). A non- uniform pulse interval clock is generated from a uniform pulse interval clock, and then used to modulate the width, and optionally the amplitude, of a drive signal to controllably drive one or more display elements of an array of display elements. A gamma correction is provided jointly with a compensation for initial nonuniformity. However, this technique is only applicable to passive-matrix displays, not to the higher-performance active-matrix displays which are commonly employed.
Existing mura and Vth compensation schemes are not without drawbacks, and few of them compensate for Voled rise or OLED efficiency loss. Those that compensate each subpixel for Vth shift do so at the cost of panel complexity and lower yield. There is a continuing need, therefore, for improving compensation to overcome these objections to compensate for EL panel degradation and prevent objectionable visible burn-in over the entire lifetime of an EL display panel, including at the start of its life.